change of visa type

My Non Immigrant Business Visa Type B will expire in about 1 month from now. I want to change to a Retirement Visa afterwards but my funds of THB 800,000 will be by then in my private bank account for just 1 month. So I have to wait for one more month till the funds are in for 2 months and I can apply for the Retirement Visa.

What should I do to cover the 1 month gap between Business Visa and Retirement Visa?

Do I have to leave the country to obtain the Non Immigrant Type O Visa for the Retirement Visa or can this be arranged within Thailand?

I would be very grateful to receive your advice and suggestions. Thank you in advance.